1. Field of the Invention
This invention relates to a capacitive-type apparatus for detecting a physical quantity such as an acceleration, an angular velocity, or a pressure.
2. Description of the Related Art
In a typical capacitive physical-quantity detection apparatus, a movable electrode and a fixed electrode are opposed to each other to form a capacitor, and a physical quantity is detected on the basis of the capacitance of the capacitor.
There are known capacitive physical-quantity detection apparatuses of various types which are designed to implement self diagnoses. In a self-diagnosis mode of operation of such a known apparatus, an electrostatic force is generated between a movable electrode and a fixed electrode to produce a state where a pseudo physical quantity is deemed to be acting on the movable electrode.
Self diagnosis is carried out by inspecting a response of an apparatus sensing portion to the pseudo physical quantity.
Japanese published unexamined patent application 5-322921 corresponding to U.S. Pat. No. 5,506,454 discloses a system for diagnosing characteristics of a capacitive acceleration sensor. In a diagnosis mode of operation of the system, a signal applicator serves as a means for applying a diagnosis signal to a fixed electrode so that a force corresponding to an acceleration is exerted onto a movable electrode. Diagnosis is carried out by inspecting a response of the sensor to the acceleration-corresponding force.
U.S. Pat. No. 5,540,095 discloses an accelerometer including two fixed electrodes opposed to a movable electrode. In a self-test mode of operation of the accelerometer, a carrier voltage level fed to one of the fixed electrodes is made different from a normal value to apply a pseudo physical quantity to the movable electrode. Self test is implemented by inspecting a response of an accelerometer sensing portion to the applied pseudo physical quantity.
U.S. Pat. No. 5,583,290 discloses a micromechanical sensing apparatus including two fixed electrodes opposed to a movable electrode. During a normal mode of operation of the apparatus, carrier signals having different center voltages are applied to the fixed electrodes respectively, and detection is made as to the capacitance of a capacitor formed by the fixed electrodes and the movable electrode. In a self-test mode of operation of the apparatus, a voltage fed to the movable electrode is made different from a normal value to apply a pseudo physical quantity to the movable electrode. Self test is implemented by inspecting a response of an apparatus sensing portion to the applied pseudo physical quantity.